How to require that a protocol can only be adopted by a specific class

Question

I want this protocol:

protocol AddsMoreCommands {
     /* ... */
}

only to be adopted by classes that inherit from the class UIViewContro

Answer 1

Because of an issue in the previous answer I ended up with this declaration:

protocol AddsMoreCommands where Self : UIViewController { 
    // protocol stuff here  
}

no warnings in Xcode 9.1